This invention relates to digital data communication systems, teleprocessing networks and the like, and in particular it relates to multiplexing schemes for introducing supervisory messages into data streams containing normal message transmissions.
As used herein, the term "supervisory message" applies broadly to any information which is utilized for monitoring and/or controlling the operations of equipments in a communication system, as distinguished from information which is utilized for normal data processing purposes without regard to the manner in which the communication equipment is functioning or malfunctioning. Supervisory messages commonly include diagnostic information sent from remote equipments to a central station and commands sent from the central station to the remote equipments in response thereto.
The term "digital" is used herein to denote information which is communicated in the form of discrete pulses, and it includes digitized analog signals as well as coded binary data.
It has been found costly and otherwise undesirable to provide special modems or special separate communication channels for handling supervisory messages. It is preferable that supervisory messages be communicated by facilities which are no more expensive and require no greater frequency bandwidth than the facilities that otherwise would be needed to handle normal message traffic in the complete absence of any supervisory messages. One way to accomplish this objective is to employ time division multiplexing whereby the time during which a communication channel is used will be divided between time intervals wherein normal messages may be sent and other time intervals that are utilized only for supervisory messages. Since the normal messages rarely use 100 percent of the time available, supervisory messages may be inserted whenever interruptions in the normal message transmissions occur. This practice does not expand the bandwidth of the communication channel nor does it affect the complexity of the normal communication protocols by the act of accommodating the supervisory messages.
In one form of time division multiplexing that has been proposed heretofore, certain regularly occurring time slices or digit spaces are reserved exclusively for the digits of supervisory messages, regardless of whether or not there are any supervisory messages to be transmitted. This practice of allocating predetermined fixed time slices to supervisory messages can be wasteful when little or no supervisory data is to be sent, and insufficient when a great deal of it is to be sent. It is just as desirable to use time economically as it is to minimize the cost of the equipment which is being used. Supervisory messages are likely to occur in random fashion at unpredictable times, not at regular intervals, and it would be inefficient to reserve regularly occurring time intervals for handling such messages, because often these intervals would not be utilized for any useful purpose. Furthermore, it is not usually necessary for the supervisory messages to preempt the channel immediately. In most, if not all, instances they can await the inevitable gaps in the normal message flow.